1. Field of the Invention
This invention relates to a modular acetabular anti-protrusio cage and acetabular cup combination for receiving the bearing insert of a hip joint prosthesis.
2. Description of the Related Art
For many years now, prostheses have been implanted in the human body to repair or reconstruct all or part of an articulating skeletal joint, such as the hip joint. The hip joint includes the femur and the pelvis, each of which has a surface for articulation against an adjacent articulation surface of the other bone. The femur has a head having a convex, generally spherically contoured articulation surface. The pelvis includes an acetabulum having a concave, generally spherically contoured articulation surface. The articulation surfaces of the femur and the pelvis form a ball-and-socket type joint.
One or both of the articulation surfaces of the hip joint may fail to perform properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface. In an artificial hip joint, a femoral head and a femoral stem can be used to replace the natural head, stem, and articulating surface of the femur, and an acetabular cup can be used to replace the natural socket and articulating surface of the acetabulum of the pelvis. The artificial femoral stem and head may be an integral unitary component or separate modular components designed to be assembled together. The prosthetic femoral head articulates directly against the artificial acetabular cup. The acetabular cup component is received and fixed within the acetabulum of a pelvis. The pelvis is prepared to receive the acetabular cup by reaming a concavity in the acetabular bone. The acetabular cup component typically has an outer surface conforming to the concavity reamed in the acetabular bone of the pelvis, and an inner bearing cavity for receiving the head of a prosthetic femoral component. The prosthetic femoral head articulates in the bearing cavity of the acetabular cup.
One known type of acetabular cup includes an acetabular shell made of a bio-compatible metal, such as titanium or a titanium or chrome-cobalt alloy, and a bearing insert made of a-material which allows the prosthetic femoral head to move about, such as a bio-compatible polymer (e.g. ultra-high molecular weight polyethylene). Some acetabular shells are attached to the acetabular bone using polymerizable synthetic cement, and others are attached to the bone using mechanical anchoring means such as screws. The shell also can be affixed by a combination of bone screws and bone cement. Still other acetabular shells can be attached to the acetabular bone using a “press-fit” shell in which the shell is inserted forcibly into the acetabular cavity. After the acetabular shell is implanted, the bearing insert is secured within the acetabular shell and the head of the prosthetic femoral component is positioned in the bearing insert.
In some instances, degenerative bone conditions or prior surgery can damage the acetabulum, and particularly its medial wall, to the extent that the acetabulum does not have the integrity to serve as a mounting platform for the acetabular shell. This condition requires a reinforcement prosthesis which is implanted within the acetabulum before the acetabular cup, and at least a portion of which receives the acetabular cup. Such a reinforcement body, sometimes known as an ant-protrusio cage, includes a main body that is at least partially cup-shaped and includes two or more integral radially extending flanges. The anti-protrusio cage is first stabilized within the acetabulum using bone cement or bone screws. Thereafter, the flanges are joined to the ilium, ischium and/or pubis to further secure the cage and to distribute forces away from the medial wall of the acetabulum. Examples of anti-protrusio cages can be found in U.S. Pat. Nos. 5,931,870, 5,871,548, 4,623,352, and 4,437,193.
While known anti-protrusio cages can be useful, they do have certain disadvantages. For example, known anti-protrusio cages often have a complicated configuration with various means for attaching fixation flanges to the body of the anti-protrusio cage. These complicated configurations typically require the use of special acetabular cups that are designed to mate with the anti-protrusio cage. Also, all existing systems involve placing a cage system in host bone and thereafter placing an acetabular component into the cage. As a result, these existing systems do not provide for optional use of a cage after placement of an acetabular cup. In other words, these existing systems do not provide a cage system designed for insertion into an acetabular cup if and when it is determined that this would be advantageous after the acetabular cup has been implanted by the surgeon.
Thus, there exists a need for an anti-protrusio cage that can be used with standard acetabular cups that are commonly used in hip replacement procedures. In particular, there is a need for an improved combination of an acetabular anti-protrusio cage and an acetabular cup that provides for maximum flexibility and does not require the use of specialized acetabular cups that are specifically designed to mate with an anti-protrusio cage. Furthermore, there is a need for a modular acetabular anti-protrusio cage and acetabular cup combination wherein the acetabular cup can be implanted first so that the stability of the acetabular cup can be determined before the cage is put in for support, if and when cage type reinforcement is still required.